Fix the broken handle() documentation

This is likely to be happening due to the mismatching function signature include
the return type between the header and source file. Two things are being fixed:

1) Dropped scope for the Handle just like with the winId case in QWidget.

2) "const" qualifier is now added for the method.

Here you can see the documentation that it is empty:
http://qt-project.org/doc/qt-5/qserialport.html#handle

Change-Id: I5e9e0732cfd746a863de523fe90e58eafa42c081
Reviewed-by: Alex Henrie <alexhenrie24@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>